A power cylinder assembly of an internal combustion engine generally includes a reciprocating piston disposed within a cylindrical cavity of an engine block. One end of the cylindrical cavity may be closed while another end of the cylindrical cavity is open. The closed end of the cylindrical cavity and an upper portion or crown of the piston defines a combustion chamber. The open end of the cylindrical cavity permits oscillatory movement of a connecting rod, corresponding with a speed of the engine, which joins a lower portion of the piston to a crankshaft, which is submersed in an oil sump. The crankshaft converts linear motion of the piston (resulting from combustion of fuel in the combustion chamber) into rotational motion.
Internal combustion engines, and in particular the pistons of such engines, are under increased stress as a result of efforts to increase overall efficiency, e.g., by reducing piston weight and/or increasing pressures and temperatures associated with engine operation. Thus, to improve engine performance, increase engine efficiency, and reduce fuel consumption, engine designs have been reduced in size in recent years. As engine size has reduced, combustion temperatures have correspondingly and generally increased. Piston cooling is therefore increasingly important for withstanding the increased stress of such operational conditions over the life of the engine.
Known piston designs include a combustion bowl facing the combustion chamber, the combustion bowl typically having a curved shape that optimizes power output of the piston during the combustion process. That is, typically the shape of the combustion bowl is selected such that the flame front grows optimally into the curved combustion bowl during each combustion event to maximize power output.
Known piston designs also typically include cooling galleries disposed approximately about a circumference of the combustion bowl, allowing for coolant fluid to pass through and remove heat during piston operation. Crankcase oil is introduced as cooling medium into a cooling gallery, and the oil removes combustion energy that passes via conduction into the piston. However, controlling the overall temperature with crankcase oil is challenging for a variety of reasons. First, knowing the actual flow rate into the cooling gallery presents its own challenges because flow distributes within an engine to each of the pistons. The flow rate out of the oil injector may be reasonably determined, but the capturing and flow within each piston may thereby not be known, though an overall flow rate to all of the pistons may be known.
Moreover, flow characteristics within each cooling gallery of all the pistons itself may not be known. The cyclical, oscillatory, or sinusoidal motion of the piston causes abrupt directional changes in the direction of travel. Such rapid directional changes result in corresponding cyclical filling and emptying steps during the overall motion of the piston. When the piston reaches top dead center (TDC), the oil in the cooling gallery travels to the top of the gallery, and when the piston reaches bottom dead center (BDC), the oil flushes to the bottom of the gallery. A piston cooling nozzle injects or otherwise introduces oil into the cooling gallery during this very rapid cyclical operation. Such operation thereby and correspondingly results in a rapid and very dynamic flushing and filling operation of coolant or oil within the cooling chamber of the piston. A volume of oil within the cooling gallery thereby is constantly changing during the dynamic, cyclical motion. The amount of oil within the cooling gallery affects the rate of heat transfer within the cooling gallery, as well, and the heat transfer coefficient in particular within the cooling chamber is very difficult to determine.
As such, manufacturers have developed different methods to determine or at least estimate the amount of heat transfer that occurs within the cooling chamber. One known method involves the use of very complex computational fluid dynamics (CFD) computer models. The computer models attempt to model and determine the overall system performance to include flow rates of the oil and the resulting heat transfer coefficients. However, CFD modeling is highly dependent on such factors as oil flow rates to each of the pistons, engine speed (as reflected in reciprocation speed of each individual piston), oil hole diameter, shape of the cooling gallery, and other physical aspects of the system. Additional factors include temperature effects, and material properties of the oil may not even be known or understood to the degree necessary to validate a CFD model. For instance, oil viscosity may not particularly be known because the temperature itself may not be known, although there are known techniques for estimating or otherwise determining the oil viscosity experimentally.
One known factor for assessing cooling in a cooling gallery of a piston is to use CFD models to estimate the amount of oil that is in the cooling gallery throughout the cyclical action of the piston. A ‘fill ratio’ is estimated in the cooling gallery, based on parameters that are input to the CDF model. The fill ratio changes throughout the cyclical action, thus in one known CFD model an average fill ratio is used, which is determined at steady-state conditions after accounting for initial transient effects.
The fill ratio may thereby be used to estimate the heat transfer coefficient that occurs within the cooling chamber. However, given the uncertainties of the CFD modeling, the instantaneous fill ratio itself may not be known, and the average fill ratio may not be accurately determined. That is, for any given flow rate into and out of the cooling chamber, the amount that actually remains within the cooling chamber during various portions of the cyclical action may not be known. The resulting heat transfer coefficient, likewise, may therefore be equally, if not more, uncertain.
Thus, many factors may converge that can cause difficulty in actually validating a CFD model of oil in a cooling gallery, and for determining how much oil is in the cooling chamber during the cyclical operation of the piston. There are therefore many factors that lead to uncertainty of determination of the heat transfer coefficient in the cooling chamber, the rate of heat transfer within the piston, and ultimately what the operating temperature of the piston is. As such, a CFD model does not necessarily provide the requisite information to estimate or otherwise understand the factors that determine the rate of heat transfer in a piston during operation.
Accordingly, there is a need for an improved method and apparatus for determining a cooling gallery fill in a piston.